Transmitting apparatus and method for a digital telecommunication system

ABSTRACT

The present invention relates to a receiving apparatus ( 1 ) for receiving signals in a digital telecommunication system and a synchronising method for synchronising such a receiving apparatus ( 1 ). The receiving apparatus ( 1 ) comprises receiving means ( 2, 3 ) for receiving a reference symbol comprising at least two repetition patterns, whereby one of said at least two repetition patterns is phase-shifted in relation to the other repetition pattern, and synchronising means ( 5 ) for synchronising the receiving apparatus ( 1 ) in the digital telecommunication system using said received reference symbol. The synchronising means ( 5 ) comprises a cross correlation means ( 16, 24 ) for cross correlating at least one of said two repetition patterns within a cross correlation window having a predetermined length. Hereby, the performance and the accuracy of a cross correlation peak detection can be enhanced for improved synchronisation.

The present invention relates to a transmitting apparatus and atransmitting method for transmitting a digital signal in a digitaltelecommunication system. The present invention is hereby particularlydirected to the generation and transmission of a reference symbol whichis used on a receiver side to achieve a time and/or frequencysynchronisation.

Digital telecommunication systems generally need a synchronisation of atransmitting side and a receiving side. The transmitting side and thereceiving side can e. g. be base stations and mobile stations of atelecommunication system, whereby the synchronisation of the timing andthe frequency of transmitted signals is usually performed in the mobilestation. To achieve a synchronisation, it is known to transmit a specialtraining sequence or a reference symbol, also called synchronisationsymbol. Such a reference symbol is usually embedded in the transmissiondata structure and regularly sent so that a synchronisation can beperformed regularly.

In FIG. 1, a general structure of a receiving apparatus is shown inorder to explain the synchronisation mechanism on which the presentinvention is based. The receiving apparatus can e. g. be a mobilestation of a wireless digital telecommunication system. Although thepresent invention essentially relates to the transmitting part of atelecommunication terminal, it is to be understood, that thetransmitting part or transmitting apparatus of the present invention canalso be a or part of a receiving and transmitting terminal.

The receiving apparatus 1 shown in FIG. 1 comprises an antenna 2 forreceiving signals from a transmitting side, e. g. a base station of awireless digital telecommunication system. The received signals 2 aresupplied to a HF means (High Frequency means) 3, which downconverts thereceived high frequency signals into the base band. The downconvertedsignals are supplied to a IQ-demodulation means, where they aredemodulated and supplied to a synchronising means 5.

The synchronising means performs time and frequency synchronisationusing a received training sequence or reference symbol, as stated above.Using the synchronisation information of the synchronising means 5, thereceived user data signals are further processed in the receivingapparatus 1, e. g. decoded by a decoding means 6 and so on, to be madeavailable in visible or audible form for a user. Usually thesynchronisation in the synchronising means 5 is performed in the timedomain.

Generally speaking, the synchronising means 5 performs a time domaincorrelation between the reference symbol (or parts of the referencesymbol) and a delayed version of the received reference symbol (or partsof the reference symbol) to identify the reference symbol (or parts ofthe reference symbol) and thus to determine the timing for thesynchronisation. Thereby, a correlation peak is calculated, which shouldcorrespond as accurate as possible to the time point of the last sampleof the reference symbol.

In order to achieve a well detectable correlation peak, the referencesymbol usually consists of a plurality of synchronisation patterns,which are repeated several times within one reference symbol period. Thesynchronisation patterns usually have the same shape or form and arethus called repetition patterns throughout the present application. Areference symbol therefore contains several repetition patterns, wherebyeach repetition pattern consists of a plurality of samples. Eachrepetition pattern has the same number of samples. Between the referencesymbol and the adjacent user data symbols, guard intervals can beinserted to avoid intersymbol interference in a multipath environment ofthe telecommunication system.

The time domain correlation of the received reference symbol in thereceiving apparatus 1 can be achieved e. g. on the basis of an autocorrelation mechanism or a cross correlation mechanism. An autocorrelation mechanism thereby does not require any knowledge about thereference symbol on the receiver side, whereby a cross correlationmechanism requires exact knowledge about the reference symbol to bereceived on the receiver side.

A known cross correlation means 7 is shown in FIG. 2. The crosscorrelation means 7 cross correlates incoming signals y(i), e. g. comingfrom the IQ demodulation means 4, within a cross correlation window of alength 16. The cross correlation window length 16 means that theincoming digital signal y(i) is cross correlated sample by sample on thebasis of a length of 16 samples. The cross correlation window length of16 samples can thereby correspond to the length of a repetition patternof the reference symbol. In FIG. 3, a reference symbol comprising 9repetition patterns is shown, whereby one repetition pattern cancomprise 16 samples. The receiving apparatus 1 knows exactly thestructure of the reference symbol to be received. A complex conjugatedversion of an expected repetition pattern is stored in the synchronisingmeans 5 and cross correlated to the received signals.

The cross correlation means 7 of FIG. 2, which has a cross correlationwindow length of 16, comprises 15 delay means 8 arranged serially. Thefirst delay means delays the incoming complex signal y(i) by one sample,which corresponds to multiplication with a factor z⁻¹. The second delaymeans delays the output of the first delay means again by 1 sample andso on. Further, the cross correlation means 7 comprises 16multiplication means 9 and a sum means 10. The delay means 8, themultiplication means 9 and the sum means 10 are arranged so that anincoming signal having a length of 16 samples is cross correlated with acomplex conjugated version of the samples of a repetition pattern. Thecomplex conjugated samples of the expected repetition pattern are e. g.stored in the synchronising means of the receiver and read outrespectively to the multiplication means 9. E. g. a first receivedsample y(0) is multiplied with a complex conjugated version of the firstsample of the expected repetition pattern, i. e. y*(0)=s_(o*). The nextreceived sample y(1) is multiplied with y*(1)=s_(l)* and so forth. Thesum means 10 adds up all the results from the multiplication means 9, sothat an output signal r(i) is obtained. The output signal r(i) of thesum means 10 is supplied to an absolute value calculating means 11 whichcalculates the absolute value of r(i) to detect a cross correlationpeak. The cross correlation means 7 and the absolute value calculatingmeans 11 shown in FIG. 2 can be comprised in the synchronising means 5of the receiving apparatus 1 shown in FIG. 1.

In FIG. 3, the cross correlation peak detection performed by the crosscorrelation means 7 and the absolute value calculating means 11 shown inFIG. 2 is explained. FIG. 3 shows three different phases of a crosscorrelation calculation of an incoming signal. In phase 1, thecorrelation window 13 of the cross correlation means 7 is located onreceived user data, which means that only user data are crosscorrelated. The user data are indicated by “??? . . . ”. Thus, no crosscorrelation peak is detected. In phase 2, the correlation window 13 isexactly matching with the eighth repetition pattern S7 of the referencesymbol 12, so that a corresponding cross correlation peak is detected.In phase 3, the cross correlation window 13 is again cross correlatinguser data “??? . . . ”, so that no cross correlation peak is detected.

The reference symbol 12 shown in FIG. 3 comprises 9 repetition patternsSO, SI, . . . , S8, which have identical shapes. Each of the repetitionpatterns comprises e. g. 16 samples, which corresponds to the crosscorrelation window length 16 of the cross correlation means 7 in FIG. 2.Of course, the number of repetition patterns in the reference symbol 12and the number of samples in each repetition pattern can be changed andadopted to the respective application.

As stated above, the cross correlation mechanism requires exactknowledge on the reference symbol to be received on the receiving side.This means, that the receiving apparatus needs to know exactly thestructure and number of repetition patterns to be able to recognise thelast cross correlation peak, which serves for a time and frequencysynchronisation. On the other hand, if one of the cross correlationpeaks is not properly detected, the synchronisation fails. In mobilecommunication environments, in which multipath fading degrades thecorrelation peak detection performance, the synchronisation performancein a known receiving apparatus of the telecommunication system is thussignificantly lowered.

The object of the present invention is to provide a transmittingapparatus and transmitting method for transmitting a digital signal in adigital telecommunication system which generate a reference symbol whichallows for an improved time and/or frequency synchronisation performanceand accuracy on the receiving side.

According to a first aspect of the invention, a transmitting apparatusfor transmitting a digital signal in a digital telecommunication systemcomprises means for preparing a reference symbol comprising a sequenceof a plurality of synchronisation repetition patterns, whereby eachrepetition pattern contains a predetermined number of samples, means fortransmitting said reference symbol as a part of said digital signal byusing OFDM (Orthogonal Frequency Division Multiplexing) modulation to areceiver side apparatus, wherein an end synchronisation repetitionpattern in said reference symbol is phase-shifted by 180° and saidreference symbol comprises a number of said synchronisation repetitionpatterns and said phase-shifted synchronisation repetition pattern ispositioned after the sequence of said number of synchronisationrepetition patterns so that the receiver side apparatus can exactlydetect a timing of a correlation peak at the end of said referencesymbol by performing a cross correlation of said synchronisationrepetition patterns. Advantageously, the transmitting apparatus of thepresent invention further comprises adjusting means for increasing thetransmission power when transmitting the reference symbol.

According to a further aspect of the present invention, a method fortransmitting a digital signal in a digital telecommunication systemcomprises the steps of preparing a reference symbol comprising asequence of a plurality of synchronisation repetition patterns, whereineach repetition pattern contains a predetermined number of samples,transmitting said reference symbols as a part of said digital signal byusing OFDM (Orthogonal Frequency Division Multiplexing) modulation to areceiver side apparatus, wherein an end synchronisation repetitionpattern in said reference symbol is phase-shifted by 180° and saidreference symbol comprises a number of said synchronisation repetitionpatterns and said phase-shifted synchronisation repetition pattern ispositioned after the sequence of said number of synchronisationrepetition patterns so that the receiver side apparatus can exactlydetect a timing of a correlation peak at the end of said referencesymbol by performing a cross correlation of said synchronisationrepetition patterns.

Advantageously, the method according to the present invention furthercomprises the step of increasing the transmission power whentransmitting the reference symbol.

According to a further aspect of the present invention, a transmittingapparatus for transmitting a digital signal in a digitaltelecommunication system comprises means for preparing a referencesymbol comprising a sequence of a plurality of synchronisationrepetition patterns, wherein each repetition pattern contains apredetermined number of samples, means for transmitting said referencesymbol as part of said digital signal by using OFDM (OrthogonalFrequency Division Multiplexing) modulation to a receiver sideapparatus, wherein an end synchronisation repetition pattern in saidreference symbol is phase-shifted by 180° and said reference symbolcomprises a number of said synchronisation repetition patterns and saidphase-shifted synchronisation repetition pattern is positioned after thesequence of said number of synchronisation repetition patterns so thatthe receiver side apparatus can perform a synchronisation process inaccordance with said synchronisation repetition patterns and exactlydetect the timing of said end of the reference symbol by performing across-correlation of said synchronisation repetition patterns.

Advantageously, the transmitting apparatus of the present inventionfurther comprises adjusting means for increasing the transmission powerwhen transmitting the reference symbol.

According to a further aspect of the present invention, a method fortransmitting a digital signal in a digital telecommunication systemcomprises the steps of preparing a reference symbol comprising asequence of a plurality of synchronisation repetition patterns, whereineach repetition pattern contains a predetermined number of samples,transmitting said reference symbol as a part of said digital signal byusing OFDM (Orthogonal Frequency Division Multiplexing) modulation to areceiver side, wherein an end synchronisation repetition pattern in saidreference symbol is phase-shifted by 180° and said reference symbolcomprises a number of said synchronisation repetition patterns and saidphase-shifted synchronisation repetition pattern is positioned after thesequence of said number of synchronisation repetition patterns so thatthe receiver side can perform a synchronisation process in accordancewith said synchronisation repetition patterns and exactly detect atiming of said end of said reference symbol by performing across-correlation of said synchronisation repetition pattern.

Advantageously, the method according to the present invention furthercomprises the step of increasing the transmission power whentransmitting the reference symbol.

According to a further aspect of the present invention, a transmitterdevice for transmitting OFDM (Orthogonal Frequency DivisionMultiplexing) signals to a receiver in an OFDM system comprises meansfor preparing a reference symbol comprising a sequence of a plurality ofsynchronisation repetition patterns, wherein each repetition patterncontains a predetermined number of samples, means for transmitting saidreference symbol as part of said digital signal by using OFDM modulationto a receiver side apparatus in said OFDM system, means for preparing areference symbol comprising a plurality of successive repetitionpatterns, whereby said reference symbol is transmitted from saidtransmitter device by using multicarriers of said OFDM system and a lastrepetition pattern of said successive repetition patterns isphase-shifted in relation to the other repetition patterns, and whereineach of said successive repetition patterns generated by said generatingmeans is composed of the same number of samples, so that saidsynchronisation repetition patterns transmitted to said receiver sidedevice are cross-correlated in said receiver side device in order toperform time and frequency synchronisation in said receiver side device.

Advantageously, in the transmitter device according to the presentinvention, the last repetition pattern of said successive repetitionpattern is phase-shifted by 180° in relation to the other repetitionpatterns. Further advantageously, the transmitter device according tothe present invention further comprises adjusting means for increasingthe transmission power when transmitting the reference symbol.

According to a further aspect of the present invention, a method fortransmitting OFDM (Orthogonal Frequency Division Multiplexing) signalsto a receiver side in an OFDM system comprises the steps of preparing areference symbol comprising a sequence of a plurality of synchronisationrepetition patterns, wherein each repetition pattern contains apredetermined number of samples, transmitting said reference symbol aspart of said digital signal by using OFDM modulation to a receiver sidein said OFDM system, preparing a reference symbol comprising a pluralityof successive repetition patterns, whereby said reference symbol istransmitted from a transmitter side by using multicarriers of said OFDMsystem and a last repetition pattern of said successive repetitionpattern is phase-shifted in relation to the other repetition patterns,and wherein each of said generated successive repetition patterns iscomposed of the same number of samples, so that said synchronisationrepetition patterns transmitted to said receiver side arecross-correlated on said receiver side in order to perform time andfrequency synchronisation on said receiver side.

Advantageously, in the method according to the present invention, thelast repetition pattern of said successive repetition patterns isphase-shifted by 180° in relation to the other repetition patterns.Further advantageously, the method according to the present inventionfurther comprises the step of increasing the transmission power whentransmitting the reference symbol.

According to a further aspect of the present invention, a transmitterdevice for transmitting OFDM (Orthogonal Frequency DivisionMultiplexing) signals in an OFDM telecommunication system comprisesmeans for generating said OFDM signals having a reference symbolcomprising a plurality of successive repetition patterns, wherein a lastrepetition pattern of said plurality of successive repetition patternsis phase-shifted in relation to the other repetition patterns, and meansfor transmitting said generated OFDM signals including said referencesymbol and transmitting data to a receiver side device, wherein each ofsaid plurality of successive repetition patterns generated by saidgenerating means is composed of the same number of samples,respectively, so that said repetition patterns transmitted to saidreceiver side device are cross-correlated in said receiver side devicein order to perform time and frequency synchronisation in said receiverside.

Advantageously, in the transmitter device according to the presentinvention, the last repetition pattern of said plurality of successiverepetition patterns is phase-shifted by 180° in relation to the otherrepetition patterns. Further advantageously, the transmitter deviceaccording to the present invention further comprises adjusting means forincreasing the transmission power when transmitting the referencesymbol.

According to a further aspect of the present invention, a method fortransmitting OFDM (Orthogonal Frequency Division Multiplexing) signalsin an OFDM telecommunication system comprises the steps of generatingsaid OFDM signals having a reference symbol comprising a plurality ofsuccessive repetition patterns, wherein a last repetition pattern ofsaid plurality of successive repetition patterns is phase-shifted inrelation to the other repetition patterns, and transmitting saidgenerated OFDM signals including said reference symbol and transmittingdata to a receiver side, wherein each of said generated successiverepetition patterns is composed of the same number of samples so thatsaid repetition patterns transmitted to said receiver side arecross-correlated on said receiver side in order to perform time andfrequency synchronisation on said receiver side.

Advantageously, in the method according to the present invention, thelast repetition pattern of said successive repetition patterns isphase-shifted by 180° in relation to the other repetition patterns.Further advantageously, the method according to the present inventionfurther comprises the step of increasing the transmission power whentransmitting the reference symbol.

It is to be noted that the use of a sequence of a plurality ofsynchronisation repetition patterns in the reference symbolsignificantly enhances the time and frequency synchronisationperformance and accuracy as compared to the provision of only a fewrepetition patterns. Further, by phase-shifting the last synchronisationrepetition pattern in the reference symbol by 180° in relation to allother synchronisation repetition patterns in the reference symbol, avery accurate and reliable phase detection on the receiver side and thusan accurate time and/or frequency synchronisation is possible.

The present invention is explained in detail in the followingdescription by means of preferred embodiments relating to the encloseddrawings, in which

FIG. 1 shows the general structure of a receiving apparatus of a digitaltelecommunication system,

FIG. 2 shows a known cross correlation means and absolute valuecalculation means for detecting a cross correlation peak,

FIG. 3 shows the cross correlation peak detection performed by the crosscorrelation structure of FIG. 2,

FIG. 4 shows the structure of a reference symbol used forsynchronisation according to the present invention,

FIG. 5 shows the cross correlation peak detection using the referencesymbol shown in FIG. 4,

FIG. 6 shows a transmitter structure according to the present invention,

FIG. 7 shows a cross correlation means and a detection means fordetecting cross correlation peaks and respective phase information onthe basis of a reference symbol as shown in FIG. 4,

FIG. 8 shows a cross correlation means and another detection means fordetecting a single cross correlation peak on the basis of a referencesymbol as shown in FIG. 4,

FIG. 9 shows a synchronisation result of the cross correlation means andthe detection means of FIG. 8,

FIG. 10. shows a further embodiment of the detection means of FIG. 6,

FIG. 11 shows a simulation result of the cross correlation means and thedetection means of FIG. 10,

FIG. 12 shows a further embodiment of a cross correlation meansaccording to the present invention together with an absolute valuecalculation means,

FIG. 13 shows a simulation result of the cross correlation means and theabsolute value calculation means shown in FIG. 12 for detecting a crosscorrelation peak,

FIG. 14 shows a further embodiment of a synchronising structureaccording to the present invention comprising a cross correlation meansaccording to the present invention and a peak threshold detection meansand a gap detection means, and

FIG. 15 shows an alternative structure to the embodiment shown in FIG.14.

FIG. 4 shows the structure of a reference symbol 14 as example for areference symbol structure to be used according to the presentinvention. The reference symbol 14 of FIG. 4 comprises 9 synchronisationrepetition patterns SO, S1, . . . S8. Each repetition pattern has alength of 16 samples S_(o), S_(l), . . . S_(I) 5. Thereby, the lastrepetition pattern S8 is phase-shifted by 180° degrees in relation tothe other repetition patterns, which means a multiplication by (−1).Thus, the last repetition pattern S8 comprises 15 samples -s_(o),-s_(l), . . . -s_(l) 5 All synchronisation repetition patterns of thereference symbol 14 have the same shape, i.e. identical content, wherebythe last repetition pattern S8 is phase-inverted by 180 ° degrees inrelation to the other repetition patterns of the reference symbol. Allother (preceding) synchronisation repetition patterns have the samephase. It is to be noted, that the reference symbol 14 can have more orless than 9 repetition patterns and that each repetition pattern canhave more or less than 16 samples.

In FIG. 5, the reference symbol 14 is shown to be embedded in a userdata sequence. The reference symbol 14 can hereby be inserted in anywanted or advantageous location within a sequence of data symbols.Between the reference symbol and the data symbols before and after thereference symbols, a so-called guard interval can be inserted in orderto avoid inter-symbol interference (ISI) in a multipath environment. Inthe time domain the reference symbol 14 has a length N and eachsynchronisation repetition pattern has a length of N_(sp) so that thereference symbol 14 consists of (N/N_(sp)) copies of the synchronisationrepetition pattern. A very efficient way of generating reference symbolsof the desired structure, e.g. in an OFDM (Orthogonal Frequency DivisionMultiplexing) transmission system, is the application of an IFFT(Inverse Fast Fourier Transformation) exploiting the properties of theDFT (Discrete Fourier Transformation) algorithm. Consequently, in orderto generate a reference symbol of length T_(s), with (N/N_(sp))synchronisation repetition patterns of length T_(s)×N_(sp)/N only every(N/N_(sp))-th DFT coefficient (every N/N_(sp)-th subcarrier in thefrequency domain) has to be modulated. At the beginning and/or at theend of a reference symbol 14, a guard interval may be inserted in orderto avoid inter-symbol interference (ISI). Hereby, the guard interval canbe formed by a cyclic extension of each symbol by copying the last fewsynchronisation repetition patterns.

The user data are indicated by “??? . . . . ”. FIG. 5 shows threedifferent phases of cross correlating a received signal having areference symbol 14, in which the last repetition pattern S8 isphase-inverted by 180°. Relating to the receiving apparatus 1 shown inFIG. 1, the data sequence of the three phases shown in FIG. 5 are forexample supplied from the IQ demodulation means 4 to the synchronisingmeans 5, whereby the synchronising means 5 is e. g. constructed as shownin FIG. 6. In phase 1, the cross correlation window 15 cross correlatesonly user data, so that no cross correlation peak is detected In phase2, the 8th repetition pattern S7 of the reference symbol 14 is matchedby the correlation window 15, so that a cross correlation peak isdetected. The relative phase of the cross correlation peak of the 8threpetition pattern S7 is also detected to be “+”. Since the 9threpetition pattern S8 is phase-inverted by 180° in relation to the 8threpetition pattern S7, the cross correlation peak detected for the 9threpetition pattern S8 has the relative phase “−” in relation to thephase of the 8th repetition pattern S7. The repetition patterns SO, S 1.. . S6 preceding the two last repetition patterns S7 and S8 have arelative phase “+”.

In phase 3 of FIG. 5, only user data are cross correlated in the crosscorrelation window 15, so that no cross correlation peak is detected. Ascan be seen in FIG. 5, by using a reference symbol structure like theone shown in FIG. 4, in which one of the repetition patterns isphase-inverted in relation to at least one of the other repetitionpatterns in the reference symbol, a relative phase information can beobtained additional to the cross correlation peak information. Thisphase information provides additional information on the position of thelast correlation peak in the reference symbol and thus a more accurateand reliable synchronisation information.

FIG. 6 shows a transmitting apparatus or transmitting device 60according to the present invention. To be precise, FIG. 6 showsimportant elements of a transmitting apparatus 60 according to thepresent invention which are necessary to explain and to understand thepresent invention. Data to be transmitted are supplied to a channelencoder 61. The output of the channel encoder 61 is supplied to areference symbol insertion circuit 62. In the reference symbol insertioncircuit 62, the reference symbols from a memory 64, where they arestored, are multiplexed by a multiplexer 63 with the data to betransmitted. The output from the reference symbol insertion circuit 62is supplied to an OFDM (Orthogonal Frequency Division Multiplexing)burst mode controller 5. The output from the OFDM burst mode controller65 is given to an inverse FFT circuit 66. The output from the inverseFFT circuit 66 is supplied to a power adjustment circuit 67. In thepower adjustment circuit 67, the transmitting power is increased when areference symbol is transmitted. The output from the power adjustmentcircuit 67 is supplied to a synchronisation repetition pattern rotation(inverting) circuit 68. The synchronisation repetition pattern rotationcircuit 68 contains a circuit 69 for extracting the last synchronisationrepetition pattern of a reference symbol, a phase shifter 70 and acombining circuit 71 combining the phase shifted last synchronisationrepetition pattern of a reference symbol with the other synchronisationrepetition patterns in the same reference symbol. The output of thesynchronisation repetition pattern rotation circuit 68 is supplied to acircuit 72 which inserts a cyclic extension into the reference symbol.Then the data stream containing the data to be transmitted as well asthe reference symbols is modulated by a modulator 73 on a radiofrequency (RF). After filtering the data to be transmitted in a filter74 the filter data are given to an RF-front-end stage 75. The referencesymbols are inserted into the data in the frequency domain to avoid thegenerally large implementation effort when inserting the referencesymbols of the data in the time domain.

The average power of the reference symbol upon transmission is lowerthan the average power of other OFDM-symbols due to the lower number ofmodulated subcarriers. Therefore, the adjustment circuit 77 is providedin order to increase the transmitting power to match the averagetransmission power of the OFDM-data symbols. This can be achieved by amultiplication of each sample of the reference symbol with a poweradjustment factor which calculates to F_(power)=√{square root over (a)}.After the power adjustment the last synchronisation repetition patternis rotated by 180°, which is realised through a multiplication by −1 inthe synchronisation repetition pattern rotation circuit 68. After thecomplex signal is converted into a real signal by the IQ-modulator 73,it is passed to the transmission RF-front-end stage 75 in order to betransmitted through an antenna over a wireless link to a receivingdevice, which is e.g. disclosed in the following figures.

In FIG. 7, a cross correlation means 16 and a detection means 19 areshown, which can be implemented in a first embodiment of a synchronisingmeans 5 of a receiving apparatus 1 of the present invention, the generalstructure of which is shown in FIG. 1. The structure of the crosscorrelation means 16 is identical to the structure of the crosscorrelation means 7 shown in FIG. 2, so that a detailed explanation isomitted. The cross correlation means 16 comprises 15 delay means 17 and16 multiplication means 18 as well as a sum means for adding the outputsof the multiplication means 18. The cross correlation window length ofthe cross correlation means 16 corresponds to the length of onerepetition pattern, which is e. g. 16 samples. A received data stream of16 samples is cross correlated with complex conjugated samples of anexpected repetition pattern stored in the receiving apparatus 1. Theoutput signal r(i) of the sum means, i.e. the output signal of the crosscorrelation means 16 is supplied to a detection means 19 for detectingthe magnitude and the phase of the signal r(i) and therefore the exactposition of the cross correlation peak of the last repetition pattern S8of the reference symbol 14 can be detected (cf. FIG. 5).

FIG. 8 shows another arrangement of the detection means. The crosscorrelation means 16 of FIG. 8 corresponds to the cross correlationmeans 16 of FIG. 7. In the example shown in FIG. 8, the detection meanscomprises a delay means 20 for delaying the output signal r(i) of thecross correlation means 16 by one repetition pattern length, which is e.g. 16 samples. The detection means 19 further comprises a subtractionmeans 21 for subtracting the output signal s(i) of the delay means 20from the output signal r(i) of the cross correlation means 16. Theoutput signal z(i)=r(i)−s(i) of the subtraction means 21 is supplied toan absolute value calculation means 22, which calculates the absolutevalue of z(i). It is to be noted, that y(i), r(i), s(i), z(i) arecomplex values so that the magnitude and the phase information iscontained in z(i). If it is assumed, that r(i) is in the part of thereference symbol, in which the phase of the repetition patterns is notphase-shifted, for example in the part SO, . . . S7 of the referencesymbol 14 shown in FIG. 4, then s(i)=r(i−16)=r(i)·e^(jφ)

z₁(i)=r(i)−s(i)=r(i)(131 e^(jφ)).

If it is assumed, that r(i) matches with the phase-inverted repetitionpattern S8 of the reference symbol 14, then s(i)=r(i−16)=−r(i)·e^(j)φ

Z₂(i)=r(i)−s(i)=r(i) (1+e^(j)φ). It can be seen that the absolute valueof z(i) is enhanced if r(i) matches with the phase-shifted repetitionpattern S8. The phase value φ has nothing to do with the phase shiftbetween the repetition pattern S7 and S8, but results from a possiblefrequency offset between the transmitter side and the receiver side.Considering the detection range of the phase change introduced by thereference symbol structure according to the present invention under theinfluence of a frequency offset between the transmitter and thereceiver, the following result is obtained: z₁(i)/z₂(i)=−j·cot(φ/2).Thus, for a none-ambiguous detection the absolute value of φ has to besmaller than π, whereby the phase value φ is the product between thefrequency offset and the duration T_(p) of one repetition pattern,φ=2πf_(offset)T_(p).

In FIG. 9, a simulation result for the absolute value of z(i) as theoutput signal of the structure shown in FIG. 8 is shown. For thereference symbol 14 comprising 9 repetition patterns, whereby eachrepetition pattern consists of 16 samples, and whereby the phase of thelast repetition pattern S8 is inverted in relation to the phase of theother repetition patterns, the cross correlation peak is expected to beat the last sample, i.e. the time point corresponding to the lastsample, of the last repetition pattern S8. As can be seen in FIG. 9, thecross correlation peak is located at sample 144, which is the correctvalue. Thus, the cross correlation means 16 and the detection means 19shown in FIG. 9 and in FIG. 8 enable a correct and efficient detectionof the cross correlation peak.

In FIG. 10, the cross correlation means 16 and another embodiment of thedetection means of FIG. 8 are shown. Thereby, the structure shown inFIG. 109 corresponds to the structure shown in FIG. 8, whereby theoutput of the absolute value calculating means 22 is supplied to anaveraging means 23 for smoothening the absolute value of z(i) outputfrom the means 22. The structure shown in FIG. 9 is particularlyadvantageous in severe noise and fading environments. The averagingmeans 23 advantageously is a moving average filter having a filterlength corresponding to the length of one repetition pattern, which isfor example 16 samples as shown in FIG. 4. The cross correlationstructures shown in FIG. 8 and 10 can e. g. be implemented in thesynchronising means 5 of the receiving apparatus 1 shown in FIG. 1.

FIG. 11 shows a simulation result for the averaged absolute value ofz(i) as the output signal of the structure shown in FIG. 10. Thedetection of the last repetition pattern having an inverted phase asshown in FIG. 4 can be seen in the transition between sample 128 andsample 144.

In FIG. 12, a second embodiment of a cross correlation means 24 isshown, which can be implemented in a synchronising means 5 of areceiving apparatus 1 of the present invention, a general structure ofwhich is e. g. shown in FIG. 1.

The cross correlation means 24 essentially has the same structure as thecross correlation means 16 shown in FIG. 7 and the cross correlationmeans 7 shown in FIG. 2. The main difference is, that the crosscorrelation means 24 shown in FIG. 12 has a cross correlation windowlength of two repetition patterns, which in the shown examplecorresponds to 32 samples, when the structure of the reference symbolshown in FIG. 4 is assumed. Thereby, the cross correlation means 24comprises 31 delay means 25, which are arranged serially andrespectively cause a delay of one sample. Further, the cross correlationmeans 24 comprises 32 multiplication means, which multiply therespective (delayed) samples of the received signal y(i) with storedpositive and negative complex conjugated values of the samples of theexpected repetition pattern. Thereby, e. g. the first sample enteringthe cross correlation means 24 is multiplied with the first complexconjugated sample s_(o)* of the expected repetition pattern. The same istrue for the rest of the samples entering the cross correlation means24, which are respectively multiplied with the rest of the stored(positive) complex conjugated samples S₁* to S₁₅*. The second 16 samplesentering the cross correlation means 24 are respectively multiplied withthe stored negative complex conjugated samples -s_(o)* to -s₁₅* of theexpected repetition pattern. Hereby, e. g. the first sample entering themeans 24 is multiplied with the negative value of the complex conjugatedfirst sample of the expected repetition pattern -s_(o)* . The same istrue for the rest of the second 16 samples entering the means 24 whichare respectively multiplied with the negative values of the complexconjugated values, namely −s₁* to −s₁₅*. It is to be noted, that thevalues s_(o), s_(i), . . . , S₁₅ of the repetition patterns SO, SI, . .. , S8, of the reference symbol 14 shown in FIG. 4 are respectively thesame. In other words, all the repetition patterns SO, S1, . . . , S8 ofthe reference symbol 14 of FIG. 4 have the same shape, except that thelast repetition pattern S8 has an inverted phase.

The outputs of the multiplication means 26 of the cross correlationmeans 24 are added up in a sum means 27, which generate an output signalz(i). The output signal z(i) of the sum means 27 is supplied to anabsolute value calculation means 28, which calculates the absolute valueof z(i). The output signal of the absolute value calculation means 28therefore provides information on the magnitude as well as on the phaseof the data signals, which are cross correlated by the cross correlationmeans 24.

A simulation result for the output of the absolute value calculationmeans 28 of the structure shown in FIG. 12 is shown in FIG. 13. In thiscase, a reference symbol similar to the reference symbol 14 shown inFIG. 4 had been used, but only with 6 repetition patterns, whereby eachrepetition pattern consists of 16 samples. The phase of the lastrepetition pattern is shifted by 180° in relation to the other precedingrepetition patterns. Thus, the position of the last sample of the lastrepetition pattern is expected to be at sample position number 96, whichis clearly visible in the simulation result shown in

FIG. 13. FIG. 13 shows clearly, that the output signal has a maximumexactly when a correct overlapping between the two repetition patternsprocessed in the cross correlation means 24 is achieved.

FIG. 14 shows an extended structure for increasing the reliability andaccuracy of the output signal of the absolute value calculation means 22of the structure shown in

FIG. 8, the averaging means 23 of the structure shown in FIG. 10 or theabsolute value calculation means 28 of the structure shown in FIG. 12.In the improved structure shown in FIG. 13, the respective output signalof the cross correlation means 24 or the detection means 19, which isthe absolute value of z(i), is supplied to a peak threshold detectionmeans 29 and a gap detection means 30. The peak threshold detectionmeans 29 detects if the absolute value of z(i) exceeds a predeterminedcross correlation peak threshold. The gap detection means 30 detects ifthe absolute value of z(i) has been below a predetermined gap thresholdbefore said detected cross correlation peak. In FIG. 13 it can be seen,that the absolute value of z(i) is zero or close to zero as long as thedata signals entering the cross correlation means are in the part of thereference symbol, where the phase of the repetition patterns is notinverted in relation to each other. Hereby, a presynchronisation can beachieved, since the detected correlation peak is only confirmed when thegap in front of the correlation peak is detected.

In other words, the gap in front of the correlation peak can be used toidentify the range for the possible position of the cross correlationpeak. Only when the peak threshold detection means 29 detects that theabsolute value of z(i) exceeds the predetermined cross correlationthreshold and the gap detection means detects that the absolute value ofz(i) has been below a predetermined gap threshold before the detectivecross correlation peak, the cross correlation peak is confirmed. In thiscase, the peak threshold detection means 29 and the gap detection means30 send respectively a positive information to a determination means 33,which can for example be an AND gate, which outputs the position of thedetected cross correlation peak only in case of a positive signal fromboth of the means 29 and 30. In front of the gap detection means 30, anaveraging means 31 and/or a delay means 32 can be located. The averagingmeans 31 can for example be a moving average filter to smoothen theabsolute value of z(i). The filter length preferably corresponds to thelength of one repetition pattern of the reference symbol. The delaymeans 32 preferably provides a delay corresponding to the length of onerepetition pattern of the reference symbol. The averaging means 31 aswell as the delay means 32 can be provided or not depending on theapplication.

FIG. 154 shows an alternative structure to FIG. 14. In FIG. 15, theabsolute value of z(i) is supplied to a peak threshold detection means29 identical to the peak threshold detection means 29 of FIG. 14. Thegap detection means 34 shown in FIG. 15 detects if the absolute value ofz(i) has been below a predetermined gap threshold before the detectedcross correlation peak and additionally detects if it has been below thepredetermined gap threshold during a predetermined gap time. In thecontrary to the gap detection means 30 of FIG. 14, which only checks onetime point before the detected cross correlation peak, the gap detectionmeans 34 of FIG. 15 checks a time period before the detected crosscorrelation peak. Identically to FIG. 14, a determination means 33,which can for example be an AND gate, determines if the output signalsfrom the peak threshold detection means 29 and the gap detection means34 are both positive and confirms the detected correlation peak to bethe required correlation peak for that case. Both structures shown inFIGS. 14 and 15 provide an increased detection accuracy and reduce thefalse alarm possibility by combined detection of a presynchronisationand a correlation peak detection. The presynchronisation, i.e. thedetection of the gap in front of a detected cross correlation peakenables to detect the range of possible synchronisation peak positions,what can be used to reduce the number of computations needed for thesucceeding synchronisations. It has to be noted, that although the crosscorrelation and synchronisation structures shown in FIGS. 7, 8, 10, 12,14 and 15 can be implemented in the synchronising means 5 of thereceiving apparatus 1 shown in FIG. 1, these inventive structures can beimplemented or used in any other receiving apparatus as long as thescope of the present invention as defined in the enclosed claims is met.

1. Transmitting apparatus for transmitting a digital signal in a digitaltelecommunication system; the transmitting apparatus comprising; meansfor preparing a reference symbol comprising a sequence of a plurality ofsynchronization repetition patterns, wherein each repetition patterncontains a predetermined number of samples, means for transmitting saidreference symbol as a part of said digital signal by using OFDMmodulation to a receiver side apparatus, wherein an end synchronizationrepetition pattern in said reference symbol is phase-shifted by 180° andsaid reference symbol comprises a number of said synchronizationrepetition patterns and said phase-shifted synchronization repetitionpattern is positioned after the sequence of said number ofsynchronization repetition patterns so that the receiver side apparatuscan exactly detect a timing of a correlation peak at the end of saidreference symbol by performing a cross-correlation of saidsynchronization repetition patterns.
 2. Transmitting apparatus accordingto claim 1, further comprising adjusting means for increasing thetransmission power when transmitting the reference symbol.
 3. Method fortransmitting a digital signal in a digital telecommunication system,comprising the steps of preparing a reference symbol comprising of asequence of a plurality of synchronization repetition patterns, whereineach repetition pattern contains a predetermined number of samples,transmitting said reference symbol as a part of said digital signal byusing OFDM modulation to a receiver side, wherein an end synchronizationrepetition pattern in said reference symbol is phase-shifted by 180° andsaid reference symbol comprises a number of said synchronizationrepetition patterns and said phase-shifted synchronization repetitionpattern is positioned after the sequence of said number ofsynchronization repetition patterns so. that the receiver side canexactly detect a timing of a correlation peak at the end of saidreference symbol by performing a cross-correlation of saidsynchronization repetition patterns.
 4. Method according to claim 3,further comprising the step of increasing the transmission power whentransmitting the reference symbol.
 5. Transmitting apparatus fortransmitting a digital signal in a digital telecommunication system; thetransmitting apparatus comprising; means for preparing a referencesymbol comprising a sequence of a plurality of synchronizationrepetition patterns, wherein each repetition pattern contains apredetermined number of samples, means for transmitting said referencesymbol as a part of said digital signal by using OFDM modulation to areceiver side apparatus, wherein an end synchronization repetitionpattern in said reference symbol is phase-shifted by 180° and saidreference symbol comprises a number of said synchronization repetitionpatterns and said phase-shifted synchronization repetition pattern ispositioned after the sequence of said number of synchronizationrepetition patterns so that the receiver side apparatus can perform asynchronization process in accordance with said synchronizationrepetition patterns and exactly detect a timing of said end of thereference symbol by performing a cross-correlation of saidsynchronization repetition patterns.
 6. Transmitting apparatus accordingto claim 5, further comprising adjusting means for increasing thetransmission power when transmitting the reference symbol.
 7. Method fortransmitting a digital signal in a digital telecommunication system,comprising the steps of preparing a reference symbol comprising of asequence of a plurality of synchronization repetition patterns, whereineach repetition pattern contains a predetermined number of samples,transmitting said reference symbol as a part of said digital signal byusing OFDM modulation to a receiver side, wherein an end synchronizationrepetition pattern in said reference symbol is phase-shifted by 180° andsaid reference symbol comprises a number of said synchronizationrepetition patterns and said phase-shifted synchronization repetitionpattern is positioned after the sequence of said number ofsynchronization repetition patterns so that the receiver side canperform a synchronization process in accordance with saidsynchronization repetition patterns and exactly detect a timing of saidend of the reference symbol by performing a cross-correlation of saidsynchronization repetition patterns.
 8. Method according to claim 7,further comprising the step of increasing the transmission power whentransmitting the reference symbol.
 9. Transmitter device fortransmitting OFDM signals to a receiver in an OFDM system, comprising:means for preparing a reference symbol comprising a sequence of aplurality of synchronization repetition patterns, wherein eachrepetition pattern contains a predetermined number of samples, means fortransmitting said reference symbol as a part of said digital signal byusing OFDM modulation to a receiver side apparatus in said OFDM system,means for preparing a reference symbol comprising a plurality ofsuccessive repetition patterns, wherein said reference symbol istransmitted from said transmitter device by using multicarriers of saidOFDM system and a last repetition pattern of said successive repetitionpatterns is phase-shifted in relation to the other repetition patterns,and wherein each of said successive repetition patterns generated bysaid generating means is composed of the same number of samples, so thatsaid synchronization repetition patterns transmitted to said receiverside device are cross-correlated in side device receiver said in orderto perform time and frequency synchronization in said receiver sidedevice.
 10. Transmitter device according to claim 9, whereby the lastrepetition pattern of said successive repetition patterns isphase-shifted by 180° in relation to the other repetition patterns. 11.Transmitter device according to claim 9, further comprising adjustingmeans for increasing the transmission power when transmitting thereference symbol.
 12. Method for transmitting OFDM signals to a receiverside in an OFDM system, comprising the steps of preparing a referencesymbol comprising a sequence of a plurality of synchronizationrepetition patterns, wherein each repetition pattern contains apredetermined number of samples, transmitting said reference symbol as apart of said digital signal by using OFDM modulation to a receiver sidein said OFDM system, preparing a reference symbol comprising a pluralityof successive repetition patterns, wherein said reference symbol istransmitted from a transmitter side by using multicarriers of said OFDMsystem and a last repetition pattern of said successive repetitionpatterns is phase-shifted in relation to the other repetition patterns,and wherein each of said generated successive repetition patterns iscomposed of the same number of samples, so that said synchronizationrepetition patterns transmitted to said receiver side arecross-correlated on said receiver side in order to perform time andfrequency synchronization on said receiver side.
 13. Method according toclaim 12, whereby the last repetition pattern of said successiverepetition patterns is phase-shifted by 180° in relation to the otherrepetition patterns.
 14. Method according to claim 12, furthercomprising the step of increasing the transmission power whentransmitting the reference symbol.
 15. A transmitter device fortransmitting OFDM signals in an OFDM telecommunication system, thedevice comprising: means for generating said OFDM signals having areference symbol comprising a plurality of successive repetitionpatterns, wherein a last repetition pattern of said plurality ofsuccessive repetition patterns is phase-shifted in relation to the otherrepetition patterns, and means for transmitting said generated OFDMsignals including said reference symbol and transmitting data to areceiver side device, wherein each of said plurality of successiverepetition patterns generated by said generating means is composed ofthe same number of samples respectively so that said repetition patternstransmitted to said receiver side device are cross-correlated in saidreceiver side device in order to perform time and frequencysynchronization in said receiver side device.
 16. Transmitter deviceaccording to claim 15, whereby the last repetition pattern of saidplurality of successive repetition patterns is phase-shifted by 180° inrelation to the other repetition patterns.
 17. Transmitter deviceaccording to claim 15, further comprising adjusting means for increasingthe transmission power when transmitting the reference symbol. 18.Method for transmitting OFDM signals in an OFDM telecommunicationsystem, comprising the steps of generating said OFDM signals having areference symbol comprising a plurality of successive repetitionpatterns, wherein a last repetition pattern of said plurality ofsuccessive repetition patterns is phase-shifted in relation to the otherrepetition patterns, and transmitting said generated OFDM signalsincluding said reference symbol and transmitting data to a receiverside, wherein each of said generated successive repetition patterns iscomposed of the same number of samples so that said repetition patternstransmitted to said receiver side are cross-correlated on said receiverside in order to perform time and frequency synchronization on saidreceiver side.
 19. Method according to claim 18, whereby the lastrepetition pattern of said successive repetition patterns isphase-shifted by 180° in relation to the other repetition patterns. 20.Method according to claim 18, further comprising the step of increasingthe transmission power when transmitting the reference symbol.